The invention relates to a mixer stage with a transistor for mixing, which is fed an input signal and an oscillator signal for mixing, wherein the input signal is fed to the mixer transistor via a frequency-selective input filter, and wherein the input impedance of the mixer transistor is smaller than the source impedance driving it at the operating frequency.
Such mixer stages are known. The input impedance of the mixer transistor in such a mixer stage is smaller than the source impedance driving it when the mixer transistor is operated using a bipolar transistor as mixer transistor in common base circuit and using a field-effect transistor as mixer transistor in common gate circuit. In mixer stages wherein the mixer transistor is operated in common base circuit or in common gate circuit, respectively, with the input impedance of the mixer transistor consequently being smaller than the source impedance driving it, the coupling coil which is used for coupling the mixer transistor to the corresponding filters is not magnetically coupled to the corresponding filter coil and is therefore also not arranged together with the filter coil on a common coil carrier. For, the opinion prevails that when the mixer transistor is operated in common base circuit or in common gate circuit, respectively, it is not possible to use the magnetic coupling between filter and transistor input customary in other basic circuits such as common emitter or source circuits because the low-ohmic input impedance of the mixer transistor excessively damping the input filter in a common base circuit or common gate circuit, respectively, thus decreasing the selectivity of the filter accordingly. On the other hand, however, a magnetic coupling filter between the filter coil and the transistor input using a coupling coil is desirable because this solution is more space-saving and also more cost efficient than a non-magnetic coupling of the mixer transistor to the filter.